In for instance the motor vehicle industry, it is desirable to keep down the weight of at least some types of threaded joints comprised in various vehicle structures. This is accomplished by utilising the maximum clamping force obtainable by a certain size of the fasteners comprised in the joint, which means that each fastener is tightened to its yield point, i.e. at the upper end of its elastic deformation range. This well known method is described in for instance U.S. Pat. No. 2,600,549.
However, when tightening a fastener to a level above its yield point, i.e. into the plastic deformation range, there is a problem to accurately determine the obtained axial load, because in that range the axial load does not increase linearly in relation to the angle of rotation.
In prior art, there has been no simple and reliable method available for obtaining accurate information about the accomplished axial load when tightening a fastener into the plastic deformation range.
According to one well known method for tightening a fastener into the plastic deformation range, the yield point is determined by detecting the significant change in torque growth obtained in that point, and by tightening the fastener over another predetermined angle from that point on. By this method, a high clamping ability of the fastener is utilised since it is tensioned above the yield point, but there is no possibility to determine the actually obtained axial load on the fastener. Occurring variations in the frictional resistance in the fastener cause a considerable scattering of the obtained axial load, not only in the yield point but also in the final shut-off point.
A well known method for determining the axial load on a fastener and described in for instance U.S. Pat. No 3,969,810, comprises measuring the change in travel time of ultrasonic waves induced axially into the fastener. This method is applicable on tightening a fastener within the elastic deformation range only, because the direct correspondence between the ultrasonic wave travel time and the axial load exists only when there is a linear relationship between the elongation of the fastener and the axial load. However, this is not the case in the plastic deformation range, where the fastener is elongated at a low axial load increase only. This means that this method, as previously described, is not useful to determine the axial load on a fastener at yield tightening.
There is also previously suggested a method for determining the obtained axial load in a fastener by measuring the change in travel time for ultrasonic waves induced into the fastener, also when tightening the fastener into the plastic deformation range. This method is based on a pre-tightening establishment of the relationship between the change in ultrasonic wave travel time and the axial load on the actual type of fastener. Based on this information, the obtained axial load is determined by measuring the change in the ultrasonic travel time and relating it to the previously established relationship between the travel time and the axial load.
This known method, however, does not take into account the variations in the frictional resistance and, thereby, the variations in axial load level obtained at the yield point of the fastener as well as in the plastic deformation range.
As mentioned above, the axial load in and above the yield point does not correspond directly to the elongation of the fastener but depends on the actual friction conditions in the fastener as well. This is due to the fact that a high frictional resistance in the fastener causes increased shearing stresses on the fastener, and a plastic deformation of the fastener takes place at a lower axial load. Accordingly, the axial load in the yield point for a certain type of fastener varies a lot since it is highly dependent on the actual friction conditions in the fastener.